This invention relates to regulated power supplies for supplying power at low voltage and high current for the filaments of high-power tubes such as those used in transmitters.
Large power vacuum or electron tubes often have filaments or cathode heaters which are heated by a flow of current in order to allow electrons to be "boiled off" for use by the tube. Power tubes by their very nature often require large quantities of electrons, and consequently large amounts of filament heating power are required. In practice, power tubes are usually operated with relatively high filament current and comparatively low voltage. This type of filament structure (low impedance) is used because, among other reasons, it results in a rugged construction, free from mechanical problems, compared with high-impedance filaments. These considerations lead to a low voltage, high current filament or heater (hereinafter termed filament) supply. The filament is normally sealed into the vacuum tube at the time of manufacture, and failure of the filament requires that the tube be discarded. Where the tubes are very expensive, as in high-power high-frequency television transmitters and the like, it is very desirable to protect the filament insofar as possible from overvoltage conditions which might result in failure. Consequently, it is desirable to regulate the power applied to the filaments.
The filament, however, being essentially a resistive element, has an impedance when cold or at turn-on which is much lower than the operating impedance. Consequently, a regulator for use as a high-power filament supply must also be "short-circuit" proof. Attempts to use semiconductor controlled rectifiers for regulation in a phase-controlled manner have encountered problems, because the turn-on surges resulting from the low impedance of the filament tend to cause failures of the controlled rectifier. A failure of the degraded controlled rectifier in a conductive mode applies uncontrolled full voltage to the filament which may in turn fail. Thus, failure of a controlled rectifier can cause failure of an extremely expensive power tube. Even when the aforementioned turn-on surges do not cause SCR failures, they pose a threat to tube life by subjecting the power tube filament to excessive current and stresses during turn-on. Also, SCR regulators produce AM switching noise. As a result, notwithstanding the advantages of regulating the filament current, the filament supply is often unregulated, in order to avoid difficulties such as those mentioned.
It is desirable in order to provide power to the filament in an efficient manner to use a three-phase power system. Thus, a current television transmitter uses unregulated three-phase AC power driving a transformer which transforms the line voltage to a low voltage. The output of the transformer in turn is coupled to a three-phase rectifier which produces pulsating direct current, which in turn is applied to the filament by a filter choke. Direct current is used on television transmitter filaments to reduce picture aberrations caused by filament supply hum, such as horizontal hum bars, etc. This arrangement is reliable, because it includes only wire, transformer iron and reliable solid-state rectifiers.
One form of regulator which has been used successfully as a filament supply in a television transmitter includes a ferroresonant transformer. The ferrorresonant transformer is operated in saturation during a considerable portion of each operating cycle. The switching of the transformer between saturation states heats the transformer, dissipating large amounts of power over that transformed for use by the filament. Consequently, the ferroresonant transformer arrangement may be less efficient than desired. Additionally, if it is desired to use a three-phase source, the ferroresonant transformer approach becomes impractical. Also, the ferroresonant approach even for single-phase operation requires large chokes and filter capacitors.
It is desirable to have a high-current low-voltage filament power supply regulator which is resistant to short-circuits in the load and has high efficiency and relatively low noise.